Datasheet

24
8116J–AVR–06/2013
XMEGA A3B
Not recommended for new designs -
Use ATxmega256A3BU
13. Battery Backup System
13.1 Features
• Battery Backup voltage supply from dedicated V
BAT
power pin for:
– One Ultra Low-power 32-bit Real Time Counter
– One 32.768 kHz crystal oscillator with failure detection monitor
– Two Backup Registers
• Typical power consumption of 500nA with Real Time Counter (RTC) running
• Automatic switching from main power to battery backup power at:
– Brown-Out Detection (BOD) reset
• Automatic switching from battery backup power to main power:
– Device reset after Brown-Out Reset (BOR) is released
– Device reset after Power-On Reset (POR) and BOR is released
13.2 Overview
The AVR XMEGA family is already running in an ultra low leakage process with power-save cur-
rent consumption below 2 µA with RTC, BOD and watchdog enabled. Still, for some applications
where time keeping is important, the system would have one main battery or power source used
for day to day tasks, and one backup battery power for the time keeping functionality. The Bat-
tery Backup System includes functionality that enable automatic power switching between main
power and a battery backup power. Figure 13-1 on page 25 shows an overview of the system.
The Battery Backup Module support connection of a backup battery to the dedicated V
BAT
power
pin. This will ensure power to the 32-bit Real Time Counter, a 32.768 kHz crystal oscillator with
failure detection monitor and two backup registers, when the main battery or power source is
unavailable.
Upon main power loss the device will automatically detect this and the Battery Backup Module
will switch to be powered from the V
BAT
pin. After main power has been restored and both main
POR and BOR are released, the Battery Backup Module will automatically switch back to be
powered from main power again.
The 32-bit Real Time Counter (RTC) must be clocked from the 1 Hz output of a 32.768 kHz crys-
tal oscillator connected between the TOSC1 and TOSC2 pins when running from V
BAT
. For more
details on the 32-bit RTC refer to the “RTC32 - 32-bit Real Time Counter” section in the XMEGA
A Manual.